1. Field of the Invention
The present invention relates to a method of controlling the characteristics of a light transmission path such as an optical fiber member or a plane waveguide path which is used in an optical communication device or the like, and an optical filter. More particularly, the invention relates to a method of forming a refractive index distribution in a light transmission path for controlling its transmission and reflection characteristics, and an optical filter having characteristics which can be dynamically controlled.
2. Description of the Background Art
In order to form a grating by a permanent fluctuation distribution of a refractive index in a silica optical fiber member for controlling its transmission and reflection characteristics, in general the interference or diffraction of a laser beam has been utilized heretofore. Alternatively, a grating of a permanent refractive index distribution has been formed by intermittently moving an optical fiber member which is irradiated with a laser beam while limiting a region irradiated with the laser beam through a slit.
FIGS. 4A to 4C schematically illustrate an exemplary method of forming a permanent refractive index distribution in an optical fiber member through interference of a laser beam. Referring to FIG. 4A, a silica optical fiber member 10 includes a core 1 which is doped with Ge and a clad layer 2 covering the core 1. An ultraviolet laser beam 21 is divided into a pair of laser beams 21a and 2lb by a beam splitter (half mirror) 22. The respective laser beams 21a and 2lb are reflected by total reflection mirrors 23a and 23b, to interfere with each other at the position of the core 1. Namely, the interfering light has an intensity distribution which periodically fluctuates along the longitudinal direction of the core 1, so that a permanent grating la having a refractive index periodically fluctuating along the longitudinal direction of the core 1 is formed by interaction between the interfering light and the Ge material of the core.
When light L is incident upon the optical fiber member 10 including the periodic permanent refractive index fluctuation distribution la, a partial light component La which is included in the light L is reflected by the grating la, so that only the remaining partial light component Lb passes through the grating la. FIGS. 4B and 4C are graphs showing the relations between wavelengths .lambda. and intensity values I in the partial light components La and Lb respectively. Namely, the permanent grating la reflects only the light component La of a specific wavelength in the incident light L, and can serve as a kind of filter.
FIG. 5 schematically illustrates an exemplary method of forming a permanent refractive index distribution in an optical fiber member through a slit. In the method shown in FIG. 5, an optical fiber member 10 including a core 1 and a clad layer 2 is partially shielded by a pair of slit masks 30a and 30b. The optical fiber member 10 is irradiated with an ultraviolet laser beam which is expressed by arrow 32 through a slit 31 between the slit masks 30a and 30b. At this time, the ultraviolet laser beam 32 has a direction of polarization which is parallel to the slit 31, as shown by arrow 33. Due to such irradiation with the ultraviolet laser beam 32, a permanent high refractive region is formed in the core 1 of the optical fiber member 10 in correspondence to the slit 31. Therefore, a permanent refractive index distribution can be formed similarly to the grating 1ashown in FIG. 4A, by intermittently moving the optical fiber member 10 along its longitudinal direction, as shown by arrow 11.
In the method utilizing interference or diffraction of a laser beam included in the conventional methods of forming permanent refractive index distributions in optical fiber members, however, the optical fiber member must be irradiated with a laser beam for at least several tens of minutes in order to form a sufficient permanent refractive index distribution, and it is difficult to stably maintain an optical system for such a longtime for forming an interference or diffraction fringe with no influence by temperature change or external vibration. Namely, the optical system for forming an interference or diffraction fringe for forming a fine grating in the core of an optical fiber member requires fine control, which disadvantageously leads to inferior controllability in the process.
On the other hand, the method of forming a permanent refractive index distribution in an optical fiber member through a slit disadvantageously requires an enormous time for forming a grating including hundreds of periodic refractive index fluctuations, although this method does not require an optical system for forming an interference or diffraction fringe. As understood from FIG. 5, only a single refractive index fluctuation can be formed by single irradiation with a laser beam through the slit.
Further, an optical filter including a permanent refractive index distribution which is formed by the aforementioned conventional method is a static filter, and it is impossible to change its characteristics after formation of the filter, except for the central wavelength of transmitted light. In order to control the transmission property of a conventional optical filter system, therefore, the optical filter member which is inserted in a light transmission path is mechanically exchanged. Alternatively, mechanical force is applied to a filter forming a permanent periodic structure of a refractive index by irradiation with a laser beam, thereby changing the central wavelength of transmitted light. However, the former method has such problems that the speed of response is so slow that selectable spectra are definite, and high density packaging cannot be attained since a filter exchanger is large-sized. On the other hand, the latter method has such a problem that the speed of response is so slow that only the central wavelength of transmitted light is controllable.